The present invention relates to a fuel cutoff control system for an engine of motor vehicles, and more particularly to a system for preventing the increase of engine speed when an accelerator pedal is released and a clutch is disengaged.
In an electronic fuel injection system, the amount of fuel supplied to an engine is calculated based on amount of air inducting into an intake passage, speed of engine, and others. The injection of the fuel is temporarily cut off during coasting when the accelerator pedal is released and the engine speed is high so as to improve the engine braking effect, emission control and fuel consumption. Numerous means for cutting off the fuel and timing thereof in various driving conditions have been proposed.
For example, Japanese Patent Laid Open No. 54-20230 discloses a system wherein the supply of fuel is stopped after a predetermined time from the time when the conditions for the cutoff are fulfilled. Therefore, fluctuation of torque is restrained to decrease a shock.
On the other hand, when a clutch is disengaged for shifting gears in a transmission after the accelerator pedal is released, the amount of air inducting into the intake passage is reduced. However, fuel adhered on walls of the intake passage is vaporized, resulting in enrichment of the air-fuel mixture. On the other hand, since the amount of fuel injection becomes extremely small at gear shifting in the electronic fuel injection system, the operation of the injector becomes inaccurate so that the air-fuel mixture becomes over lean.
In order to prevent the combustion from damping which occurs as a result of such an irregular concentration of the air-fuel mixture, Japanese Patent Laid Open No. 58-195030 proposes to cut the fuel when the amount of change of the inducting into the intake passage air per one revolution of a crankshaft of an engine is under a predetermined amount during the disengagement of the clutch.
However, in an engine having a large intake manifold volume downstream of a throttle valve, the amount of air inducting into a combustion chamber does not immediately reduce after the release of the accelerator pedal. Particularly, in an electronic fuel injection system wherein the amount of fuel is determined depending on pressure of the intake passage, the reduction of supplied fuel is retarded, corresponding to the delay in reduction of the air inducting into the combustion chamber, thereby delaying in decreasing the engine torque. Accordingly, if the clutch is disengaged immediately after the throttle valve is rapidly closed, the engine speed quickly and largely increases due to the non-load condition.
Although the fuel supply is stopped for coasting at high engine speed, neither of the above-mentioned prior arts prevents the speed up of the engine.